The present invention relates to tuning control systems used in television receivers including video tape recorders (VTRs) and AM FM radio receivers, and more particularly, to such apparatus which, in turn, includes an analog to digital converter.
Recently, a tuning control system with a controllable electronic tuner has its main portion made of multi-function ICs. The IC of one chip is used not only for control of tuning, but also for control of on and off operations of a power supply of a television receiver or for adjustment of acoustic volume. The multi-function IC which serves its central role is desired to be used in many kinds of apparatus in terms of productiveness and cost.
A conventional tuning control system will now be described with reference to the drawings. FIG. 1 is a block diagram of a tuner of a conventional television receiver. A reference numeral 1 denotes an automatic frequency regulator (AFT) of the television receiver. The AFT voltage signal corresponding to the output signal of the well-known AFT 1 is inputted to an analog to digital (A/D) converter 3 of a tuning control IC 13 to be converted into an AFT digital signal, which is then input to a central processing unit (CPU) 5 where the digital AFT signal is processed by control software stored in a read only memory (ROM) 6. The resulting data signal is outputted from an output port 7 connected to CPU 5 so as to correct the reception frequency of an electric tuner 9.
A DC power source voltage E.sub.0 has series connected voltage dividing resistors 11 and 12 having resistance values R.sub.11 and R.sub.12, respectively. If R.sub.11 =R.sub.12, a voltage signal of E.sub.0 /2 is applied to the A/D converter 4. The voltage levels set by the resistors 11 and 12 correspond to three operational modes, as shown in FIG. 3.
If a key switch 15 which selects one of television and video inputs is pressed so as to select a video mode, the input signal voltage to the input port 14 drops from E.sub.0 (volts) to 0 (volts). The control software stored in the ROM 6 is programmed so as to be expressed by a flowchart shown in FIG. 2. As will be understood from the flowchart, when the input voltage level to the input port 14 becomes 0 volts, CPU 5 takes the data signal from the A/D converter 4. Applied to the input of the A/D converter 4 is a voltage signal of E.sub.0 /2 (volts) indicating that the number of video input terminals is 1 (unity). Therefore, a binary operational-mode selective signal (TV mode: H, and VTR mode: L corresponding to the initial set mode L) when the number of video input terminals is 1 (unity) is output to the output port 8 to thereby control the television/video input switching circuit 10. The operational mode signal is circulated in the order of a television mode.fwdarw.a video mode.fwdarw.the television mode. While the operation of the tuning control system having a single video input terminal has been described, the operation of a tuning control system having two video input terminals is as follows. The initial setting by the resistors 11 and 12 is made such that the set voltage is between 2E.sub.0 /3 and E.sub.0. Under such conditions, when the key switch 15 is pressed, processing is performed as in the initial setting mode mentioned above, and a mode selective signal which circulates the operational mode is outputted from the output port 8 in the order of television mode.fwdarw.video mode.fwdarw.television mode. In this way, the station selecting apparatus is able to set three kinds of initial modes: a non-video input terminal mode, a single-video input terminal mode, and a two video input terminal mode in accordance with voltage levels set by combinations of resistors 11 and 12. In addition, the apparatus is usable in three kinds of television receivers.
However, in the conventional device, at least two A/D converters are employed in the tuning control IC; namely, the A/D converter for reception of the AFT voltage signal and the A/D converter for reception of a mode setting voltage signal must be provided. The A/D converter occupies a relatively large area when it is designed so as to include IC circuits, so that only provision of many A/D converters would bring about a large-scale IC and increase the cost. There is a known multiplexing technique which realizes an apparatus using a single A/D converter. In this case, however, the number of IC pins is required to increase only for multiplexing control purposes.